Road cars don't always have front brakes larger than rear - I have two cars with identical sized brake discs front and rear

Generally speaking if the weight is towards the rear of the car (the two above are approx. 60% rear) - then the rear discs will need to do more of the work than with a front weight bias - especially if the car has a low CofG which helps reduce load transfer to the front under braking.

In truth I expect you will find that the front/rear discs on an F1 car are very similar in size - perhaps the caliper pistons will differ as the load transfer forward is probably quite significant even allowing for the very low CofG these cars have.

In roadcar its also not that odd to have rear disc actually bigger...I think certain generation of some minivan has that. This is diametrically though, as in some of those cases the front disc are vented while the rear may be a solid disc.

There are other ways in which the brake force is distributed as well. In road car you are dealing with a single master cylinder, where as race cars usually have a front and a rear, which likely different MC to caliper Piston diameter ratio, which will "pre" bias the brake force as well(aside from a brake balance bar). F1 car can reach 5G underbraking, it is not unusual to assume that their front brake will be doing a lot of the work, but at the same token they also have significant downforce to maximize grip at all 4(which contributes to the ability of car to brake at 5G), but which is being shed as the car slows, which is probably why Ferrari is rumored to have a system that adjusts brake bias mechanically at speed, to redistribute brake force as the grip diminishes...

F1 cars typically are very well balanced front-to-rear (as well as side-to-side) during braking, with regards to both mechanical and aerodynamic forces. So each tire should require fairly equivalent braking capacity.

F1 cars use CRC (Carbon Reinforced Carbon) discs and pads. The actual physical size of the rotors (ie. the OD and thickness) and thickness of the pads is determined by mainly two criteria: First, since the pads and rotors both wear at similar rates, there must be enough pad and rotor disc thickness to safely last an entire race without becoming too thin. Second, the rotor disc itself is not sized by braking torque requirements. It is sized to provide a thermal mass that is capable of absorbing and dissipating the heat generated during braking, without exceeding the temperature limits of the CRC rotor materials. Even though CRC has a very high temperature limit (>1500degF), it will still suffer oxidation degradation above a certain temperature.

The brake calipers themselves are likely the same front-to-rear, since the clamping force needed is fairly similar. And any small differences can be compensated for by the driver using the brake bias adjuster. Each caliper itself actually has different sized pistons from leading edge to trailing edge, to counter the effects of the approach action friction on the pads. So what that means is that there are mirror image calipers from left side to right side.

Regards,
Terry

"Q: How do you make a small fortune in racing?
A: Start with a large one!"

Traditionally the brake disc diameters are fixed by the regulations near 300mm or by the regulations regarding rim size. In 2009 the maximum disc diameter was decreased from 305mm to 278mm. 305mm is about 12" which leaves roughly 1/2" clearance between the disc and the 13" rims for the caliper. For all the reasons listed by everyone else it is very likely (and in 2008 it was compulsory) that the front and rear brake discs have essentially the same diameter.

2008 Technical Regs wrote:
11.3.2 No brake disc may be more than 28mm thick with maximum and minimum diameters of 305mm and 300mm respectively.

2009 Technical Regs wrote:
11.3.2 All discs must have a maximum thickness of 28mm and a maximum outside diameter of 278mm.